It has been great problems that the use of couplers of the 5-pyrazolone type for formation of magenta dyes, which has been in conventional practice, involves secondary absorption in the dye formed, a detrimental factor in color reproduction, in the region of 430 nm, and yellowing (hereinafter referred to as "Y-stain") of the non-colored portion under influence of heat and/or moisture
To solve this problems it has been proposed to use various magenta couplers, for example, pyrazolobenzimidazoles described in British Patent No. 1,047,612, indazolones in U.S. Pat. No. 3,770,447, and pyrazoloazoles in U.S. Pat. No. 3,725,067, British Nos. 1,252,418 and 1,334,515, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as "Japanese Patent O.P.I. Publication") Nos. 59-162548/1984 and 59-171956/1984, etc. Indeed by the use of these couplers the secondary absorption of the dyes in the region of 430 nm has become reduced to a very slight degree, and the Y-stain caused by heat and/or moisture is observed but very slightly in the products.
Dyes produced by using said pyrazoloazole-type couplers have excellent properties as mentioned above, especially markedly when used in color papers.
However, differring from the 5-pyrazolone-type couplers which have been in conventional use, the use of couplers based on pyrazoloazole is accompanied with a tendency to make the gradation of the images soft at the toe portion of the characteristic curve. In color photosensitive materials, this tendency shows as disharmony of the gradations among the red-sensitive, green-sensitive and blue-sensitive layers, in many cases with prominence of magenta at the highlight, and brings about a phenomenon generally referred to as `pinkish highlight` in the case of color prints. Since man's sense of sight is capable of discriminating such delicate inconsistency especially in low density areas, this tendency to `pinkish highlight` is seriously detrimental to the commercial value of the product.
There have been various approaches to establishment of methods to control the gradation of images in silver halide photographic materials. Some of these approaches which sought general applicability are by changing the coating quantities of the silver halide or coupler, by changing the quantitative ratio of the silver halide to the coupler in the coating, by mixing two or more kinds of silver halide emulsions which differ from each other in grain size or sensitizing method as described in Japanese Patent O.P.I. Publication Nos. 50-71320/1975, 53-44016/1978, 56-78831/1981, 57-58137/1982, 57-150841/1982, 57-178235/1982 and 58-14829/1983, by chemically sensitizing or color sensitizing the silver halide grains by the use of additives, treating condition, and the like especially selected to make the gradation at the toe contrasty, and by the use of a water-soluble salt of rhodium as an additive in forming silver halide grains as described, for example, in Japanese Patent O.P.I. Publication Nos. 52-11029/1977 and 52-18310/1977, British Patent No. 1,535,016, and U.S. Pat. No. 3,448,709.
The methods devised by these approaches, such as the examples referred to above, however; involve possibilities that the gradation may become contrasty even at the shoulder of the characteristic curve as well as at the toe portion and moreover the photographic properties in other respects may be affected as well.
In other approaches, it has been proposed to use certain special compounds as additives for the purpose of redressing the fault of the gradation at the toe portion, for example, as described in Japanese Patent O.P.I. Publication Nos. 61-267050/1986, 62-169159/1987 and 62-172358/1987, but not only is the effect insufficient but the use impairs the image preservability and especially markedly the light fastness.
On the other hand, rapidness has been such a requirement in processing of photographis that it is no longer unusual for processing orders brought in to have to be returned to the customers within a matter of several hours. To meet this requirement for rapid processing, adaptation of both the photosensitive materials and the processing condition has been under consideration. For color development, it has been considered to use high temperatures, to raise pH, to increase the concentration of color developing agents, to use additives such as development accelerator, and so on. Examples of development accelerators for this purpose are 1-phenyl-4-pyrazolidone mentioned in British Patent No. 811,185, N-methyl-p-aminophenol mentioned in U.S. Pat. No. 2,417,514, and N,N,N",N'-tetramethyl-p-phenylenediamine mentioned in Japanese Patent O.P.I. Publication No. 50-15554/1975. These methods, however, are not quite satisfactory in speeding up the processing and even impair, in many cases, the photographic properties, for example, intensifying the liability to fogging.
In this connection, it is known that the shape, size and composition of the silver halide grains in the silver halide emulsion used in a photosensitive material have a great influence upon the developing speed, and the like. Especially the influence of the halogen composition is known to be outstanding and the developing speed is shown to be greatest when a silver halide with a high silver chloride content is used.
A serious problem arises, however, when a pyrazoloazole-type coupler is used in an emulsion containing silver chloride in a high ratio, since this combination makes the softness of the gradation at the toe portion more conspicuous. Accordingly, development of some technique to improve pyrazoloazole-type couplers in gradation characteristic at the toe portion without impairing the other photographic properties has been awaited in the hope of making them applicable to photosensitive materials for rapid processing.